1. Field of the Invention
The present invention relates to the field of machining crankshafts and more particularly to modifications ensuring the localization of machining centerings (part axis) under the best conditions.
2. Discussion of the Background
Crankshafts are mechanical parts presented in the shape of shafts which ensure the transformation of an alternative rectilinear movement of the set piston-connecting rod of a thermal motor in circular movement. These parts are classically executed by forging or casting before being machined precisely to integrate inside a thermal motor crankcase.
In the example of a thermal motor with four cylinders, the crankshaft comprises five bearings by the means of which it is in pivoting link with the crankcase, four crankpins arranged parallel with the rotation axis of the crankshaft and being used to drive the four connecting rods and four counterweights ensuring a balance of the crankshaft mass in relation to its rotation axis with the four crankpins and the eight arms which shift them from the rotation axis of the crankshaft.
As machining is expensive, it is therefore essential to limit duration and frequence and thus to keep a maximum of raw surfaces on the crankshafts while still respecting the geometrical and dynamic constraints voiced by the motor vehicle manufacturers.
The geometrical constraints are machining precision constraints which must be as precise as possible so as to optimise the pivoting links of the crankshaft with the crankcase or the connecting rods. The crankshaft being in origin a forged or cast part, the reference surfaces, that is to say centering points, must be as precise as possible to assume an optimum repetition of precision of position, that is to say an identical position of the crankshaft from one machining station to another.
The dynamic constraints, on their part, concern the balancing of the crankshaft which, during its rotation, must not create a moment of inertia outside the rotation axis of the crankshaft. The mass (mass of the crankpins, mass of the counterweights) and above all the surfaces staying raw after machining must be sufficiently balanced so that during the rotation of the crankshaft, the unbalance remaining is minimised. The unbalance remaining, that is to say a fault in the balancing of the crankshafts can, if it is too great, bring on early wear of the components in contact with it and cannot ensure a good transmission of movement, thus bringing about losses in output as well as excessive fuel consumption. Also, remaining unbalance generates a vibratory effect which, if it is too great can cause of lack of comfort in the driving of the vehicle.
The existing methods for machining a crankshaft offer then to ensure at the start of their machining line, a reference centre depending on a dynamic mass measurement effective from the raw crankshaft.
Classically, after the dynamic mass measurement the centering points defining the reference axis are machined so that the unbalance zone and thus the principal inertia axis are situated in the counterweights zone, so that during the passage of the xe2x80x9cbalancing devicexe2x80x9d at the end of the machining line, the machined crankshaft can be balanced by removing a minimum of matter in drilling the counterweights. It is obvious that during the passage of the crankshaft on the balancing device at the end of the machining line, this balancing device measures its unbalance by driving it in rotation with cylindrical surfaces coaxial with the rotation axis of the crankshaft itself. As mentioned in the prior art description of the European patent Nxc2x00 334 298, these cylindrical surfaces have been previously machined during the earlier machining stages using the centering points which were defined by machining after dynamic mass measurement.
Nevertheless, the raw surfaces of the crankshaft prevent a precise enough dynamic mass measurement to answer to the ever greater quality of the machining criteria required by motor vehicle manufacturers. It is particularly difficult to ensure a good repetition of position by making the ends of a crankshaft hold by the jaw of a chuck to drive it in rotation so as to measure dynamically the mass then in placing those ends on numerised supports so as to direct the crankshaft to means of centering points drilling as the jaws are tight on a raw surface, and thus imprecise. Similarly, even though the principle of numerised supports is the technical solution most adapted to the positioning of a part, the capacities of this positioning are not fully exploited as the surfaces with which the said supports are in contact are not regular. Not only do the numerised supports take into account an imprecise dynamic measurement because it has been executed on irregular surfaces, but also the numerical supports direct the crankshaft in resting also on raw surfaces, which has for consequence a bad materialisation of the inertia axis by centering points drilling.
Furthermore, this lack of precision in locating the principal inertia axis and therefore the reference axis increases the remaining unbalance as the crankshaft progresses little by little in the machining line resulting from the lack of precision in the repetition of precision of position so that during the passage in xe2x80x9cbalancing devicexe2x80x9d the removal of matter on the counterweights at the end of the production line of the crankshafts becomes a long and expensive process.
So, even though the passage of the raw crankshaft in a device of dynamic measurement enables to better evaluate the unbalancing zone and ensures a better geometrical centering, this method on its own is impaired by its relative imprecision.
On the basis of these findings, the applicant has questionned the machining line of classical crankshafts and conducted research to lead to finding a method for producing crankshafts which ensures better locating of the unbalance so as to determine, with the maximum of precision, the reference axis of the crankshaft and thus ensuring an optimum repetition of precision of position. This research has led to a particularly original and judicious method of production reaching a precision never before reached in this field without incurring prohibitive additional cost in the production of crankshafts.
According to the principal characteristic of the invention, the method for machining a crankshaft of the type comprising the operations for dynamic mass measurement and of centering points drilling at the start of the machining line, is remarkable in that it consists in pre-setting by machining of the raw crankshaft the reference surfaces before the operation of mass dynamic measurement. This characteristic is particularly advantageous as until now, and as described in the prior art, the dynamic unbalance measurement has been taken from the raw surfaces of the crankshafts. It will therefore be easier to materialise the inertia axis from the machined reference surfaces and not the raw ones. The definition by machining of a fictitious geometrical axis even if it is erroneous enables to take a fixed reference during dynamic mass measurement and thus ensures an optimum configuration of the numerised supports during centering points drilling. Also, the pre-setting of reference surfaces guarantees a good repetition of precision of position between the dynamic mass measurement station and the centering points drilling one materialising the reference axis, this repetition of precision of position was not ensured in the prior art methods as the driving of the crankshafts was done directly on a raw part.
The reconsideration of the machining line by the applicant, realized by this first claim, constitutes a break from the usual reasoning of the professionals as it advocates machining before execution of the reference surfaces and thus with the support and gripping points on the raw surface of the crankshaft. This new technique detaches itself from the ones proposed in the prior art in which the reference surfaces were machined on the crankshaft before it was balanced but at an advanced stage of its final formatting into shape, that is to say that the machine machining such reference surfaces before balancing could use surfaces already machined as support and gripping points. In view of the lack of precision of the first balancing (as executed on a raw surface), the realisation of reference surfaces for final balancing taking support on surfaces machined from a badly situated geometrical axis, also lacks precision and this leads to a long and expensive operation of balancing (when this is possible).
According to a particularly advantageous characteristic of the invention, the method for machining crankshafts is remarkable in that it consists in pre-setting the said reference surfaces by machining both ends of the crankshaft before the operation of dynamic mass measurement. The choice of place of reference is particularly judicious by the fact that the ends of the crankshafts are surfaces which are easy to machine compared with the other bearings which find themselves inserted between the arms or between the counterweights.
According to another particularly advantageous characteristic of the invention, the method consists in pre-setting the reference surfaces by trimming the cylindrical and plane surfaces of both ends of the crankshaft. The use of the trimming operation is particularly advantageous by the fact that it takes away the need to turn the crankshaft which, as a raw part, would thus give a bad engagement and a bad balancing in the case of an engagement by one of its ends in a chuck for example. Also, the trimming operation enables to take support points on the crankshaft and thus to integrate in the whole of the indications of dimensions constituting the machining assembly of this trimming operation, the indications of dimensions of the raw crankshaft so as to respect the criteria of space needed and the movement of the crankshaft in the crankcase or of passage with the piston, such as fixed by the car manufacturer.
According to another particularly advantageous characteristic of the invention, the method consists of placing the crankshaft in a dynamic measurement device, after the machining operation of the reference surfaces and ensuring the rotation of the crankshaft by driving by adherence on the aforesaid machined cylindrical surfaces. It is indeed possible thanks to the pre-setting by machining of the reference surfaces to use such a means of setting into motion to make the crankshaft rotate on the fictitious geometrical axis materialised by the pre-machined surfaces.
According to another particularly advantageous characteristic of the invention, during dynamic mass measurement the diameter of the cylindrical reference surfaces having just been machined on the crankshaft, is measured so that the said diameter enters in the positioning parameters of the numerised supports used to position the crankshaft during the centering points drilling operation. This additional parameter, other than the fact that it brings a better precision in the definition of the reference axis used to position the crankshaft on a large part of the machining line, enables also to take into consideration the wear of the tools used for the machining operation of the reference surfaces before balancing.
In conceiving this new method, the applicant aims by an action of that same method in suitability with the improvement of the precision and of the state of the surface of the crankshafts coming out of forging or of casting coupled with an evolution of the criteria of the remaining unbalance of the crankshaft at the end of machining, to do away with the final balancing station classically present at the end of the crankshaft machining line.
The fundamental concepts of the invention having just been detailed hereinabove in their most elementary form, more details and characteristics will come out more clearly when reading the description hereinafter using as a non limitative example and having regard to the attached drawings, an illustration of a method for machining crankshafts in accordance with the invention.